In an internal combustion engine for vehicles, fuel vapor treatment device for collecting the fuel vapor generating in the fuel tank is provided to prevent the fuel vapor from being released to the atmosphere. The fuel vapor treatment device includes a collector (or a canister) for collecting fuel vapor generating in the fuel tank; a purge line that introduces purge gas into an intake passage of the engine, the purge gas being a mixture of fuel vapor released from the collector and air; and a purge valve provided in the purge line to adjust flow rate of the purge gas.
Because of a limitation of the amount of fuel vapor collected, “purging process” is performed. In the purging process, the purge valve is opened to release fuel vapor from the collector, and then purge gas, i.e., a mixture of the fuel vapor and air, is introduced into an intake passage via the purge line to combust the purge gas in a combustion chamber while the engine operates. By performing such a purging process, the performance of collecting fuel vapor by the collector is recovered.
When the purging process is performed, in addition to the fuel injected from the fuel injection valve, fuel vapor contained in the purge gas is also introduced in the combustion chamber of the engine. Thus, in the fuel injection control during the purging process, fuel injection amount is reduced depending on the amount of the fuel vapor contained in the purge gas thereby reducing or preventing the fluctuation in the air-fuel ratio.
After the purging process starts, it takes some time for the amount of purge gas corresponding to the opening degree of the purge valve to flow into the combustion chamber. More specifically, as illustrated in FIG. 4, when the purge valve is opened at time t1, purge gas that has passed through the purge valve starts flowing into the combustion chamber at time t2 after some degree of delay time. Then, the inflow amount of purge gas flowing into the combustion chamber is gradually changed at a certain degree of change. After some degree of response period goes by, purge gas starts flowing into the combustion chamber in an amount corresponding to the opening degree of the purge valve, at time t3.
As described above, there are delay time and transportation delay of purge gas depending on the degree of change. Thus, in order to reduce the fuel injection amount depending on the amount of the fuel vapor contained in the purge gas, such transportation delay of purge gas must be taken into consideration. Although FIG. 4 illustrates an example when the amount of purge gas is increased after the opening of the purge valve, the transportation delay also occurs when the amount of purge gas is decreased after the closing of the purge valve.
To address this, for example, in an apparatus described in Japanese Patent No. 3582137, the amount of purge gas that flows into the combustion chamber is estimated based on the passed amount of purge gas that has passed through the purge valve and a formula that models the transportation delay. Also, since the velocity of intake air flowing into the combustion chamber becomes greater as the engine rotation speed is higher, the time of the transportation delay is likely to become shorter. Thus, in the case when the transportation delay is estimated by the formula, by setting a compensation value for compensating the transportation delay (i.e., a value for estimating the amount of purge gas flowing into the combustion chamber based on the passed amount of purge gas that has passed through the purge valve) based on the engine rotation speed, the amount of purge gas flowing into the combustion chamber can be estimated.
Generally, as the engine rotation speed becomes higher, internal pressure of the intake passage and velocity of intake air flowing into the combustion chamber become greater. In this way, when the increase in the internal pressure and the increase in the velocity of intake air are correlated, i.e., when the internal pressure and the velocity of intake air are positively correlated, the compensation value of purge gas for compensating the transportation delay can be set based on the engine rotation speed.
However, in an internal combustion engine including a variable lift device for changing a maximum lift of the air intake valve 19 and in an internal combustion engine including an exhaust gas recirculation mechanism into the intake passage, it sometimes happens that the internal pressure and the velocity of intake air are not positively correlated. It has been revealed that the setting of the compensation value for compensating the transportation delay based on the engine rotation speed cannot be made accurately in such engines.
When the compensation value of purge gas cannot be set accurately, the amount of purge gas flowing into the combustion chamber cannot be estimated, either. This makes it impossible to accurately estimate the amount of fuel vapor in the purge gas flowing into the combustion chamber and ultimately to accurately reduce the fuel injection amount during the period when purging process is performed, either.
Accordingly, there is room for improvement in estimating accurately the amount of purge gas flowing into the combustion chamber in which the internal pressure and the velocity of intake air are not positively correlated.